1. Field of the Invention
The invention relates to a neutral start switch that detects a range position for the purpose of controlling an automatic transmission and opens and closes an operating circuit of a vehicle-equipped appliance in accordance with each range position.
2. Description of the Related Art
A typical automatic transmission used in a vehicle restricts the range of gear speeds achievable in accordance with a range position selected by operating a shift device, and changes the gear speed within the restricted range, in accordance with the vehicle speed and the throttle opening. For such gear speed changing control, a sensor switch for detecting the range position selected as described above is provided for the automatic transmission. The sensor switch is operated in cooperation with the shift device. The switch not only detects the range position, but also drives a starter motor and turns on a reverse lamp, both of which are vehicle-equipped appliances. The switch is generally termed a neutral start switch because when the shift device is at the neutral position, the switch closes the starter motor driving circuit and thereby allows the starter motor to be started.
The number of range positions in the automatic transmission is increasing along with the increase in the number of the gear speeds. In many of the latest automatic transmissions having five forward gear speeds, seven range positions are provided, that is, the "P" (parking) position, the "R" (reverse) position, the "N" (neutral) position, the "D" (drive) position, the "3" (third) position, the "2" (second) position and the "L" (low) position. If contacts are provided corresponding to the increased number of range positions so as to detect each range position, the neutral start switch becomes large in size, thereby degrading vehicle installability. Therefore, in a conventional neutral start switch technology as disclosed in Japanese Patent Application Laid-Open No. HEI 6-341541, the range position is determined by an electronic control unit using combinations of on-off signals in accordance with the closed and open states of four position stationary contacts (in this specification, referred to as "contact PA", "contact A", "contact B" and "contact C" to distinguish the position contacts).
In the application of this conventional technology to a neutral start switch that also opens and closes the starter motor driving circuit and the reverse lamp lighting circuit, a structure can be considered in which stationary contacts separate for each of those circuits are arranged together with the position stationary contacts along a single movable contact. However, the adoption of the common movable contact for the three different kinds of stationary contacts causes a problem due to the functional differences among them, that is, the effect of sparks caused by the closing and opening of the contacts. That is, the starter contacts do not produce sparks when closed or opened, since the closing or opening of those contacts does not involve the electrical conduction or discontinuation thereof. The position stationary contacts do not produce significant sparking since the current conducted or shut down at the time of the closing or opening of the contacts is as small as several ten milliamperes, and the voltage is also low compared with the battery voltage of 12 V. In contrast, as for the reverse lamp contact, the closing and opening thereof involves the conduction and discontinuation of electricity at the battery voltage, and the current is as large as about 5 A. Therefore, the closing and opening of the reverse lamp contact produces large sparks.
Large sparks heat and carbonize the stationary contacts. Carbon powder will be scraped off one stationary contact and transported to another stationary contact by the movable contact, thereby reducing the conductivity of the contact. Furthermore, the movable contact also undergoes spark wear so that the stability of contact with the stationary contacts is reduced. Such reduction in the conductivity and the contact stability does not become a significant problem with the reverse lamp circuit. However, with regard to the position circuit, which operates by weak currents, a reduced conductivity or a reduced contact stability increases the possibility of signal failure. With regard to the starter circuit, which operates by large currents, the possibility of a start failure due to a reduction in the starting current increases.